1SEM_POST(3P)               POSIX Programmer's Manual              SEM_POST(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       sem_post — unlock a semaphore
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SYNOPSIS

15       #include <semaphore.h>
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17       int sem_post(sem_t *sem);
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DESCRIPTION

20       The sem_post() function shall unlock the semaphore referenced by sem by
21       performing a semaphore unlock operation on that semaphore.
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23       If  the semaphore value resulting from this operation is positive, then
24       no threads were blocked waiting for the semaphore to  become  unlocked;
25       the semaphore value is simply incremented.
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27       If  the  value  of the semaphore resulting from this operation is zero,
28       then one of the threads blocked waiting  for  the  semaphore  shall  be
29       allowed  to  return  successfully  from its call to sem_wait().  If the
30       Process Scheduling option is supported,  the  thread  to  be  unblocked
31       shall  be chosen in a manner appropriate to the scheduling policies and
32       parameters in effect for the blocked threads. In the case of the sched‐
33       ulers  SCHED_FIFO  and  SCHED_RR,  the  highest priority waiting thread
34       shall be unblocked, and if there is  more  than  one  highest  priority
35       thread  blocked  waiting  for  the semaphore, then the highest priority
36       thread that has been waiting the longest shall  be  unblocked.  If  the
37       Process  Scheduling  option  is  not defined, the choice of a thread to
38       unblock is unspecified.
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40       If the Process Sporadic Server option is supported, and the  scheduling
41       policy is SCHED_SPORADIC, the semantics are as per SCHED_FIFO above.
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43       The  sem_post()  function shall be async-signal-safe and may be invoked
44       from a signal-catching function.
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RETURN VALUE

47       If successful, the sem_post() function shall  return  zero;  otherwise,
48       the function shall return -1 and set errno to indicate the error.
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ERRORS

51       The sem_post() function may fail if:
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53       EINVAL The sem argument does not refer to a valid semaphore.
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55       The following sections are informative.
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EXAMPLES

58       See sem_timedwait().
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APPLICATION USAGE

61       None.
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RATIONALE

64       None.
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FUTURE DIRECTIONS

67       None.
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SEE ALSO

70       semctl(), semget(), semop(), sem_timedwait(), sem_trywait()
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72       The  Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Syn‐
73       chronization, <semaphore.h>
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76       Portions of this text are reprinted and reproduced in  electronic  form
77       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
78       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
79       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
80       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
81       event of any discrepancy between this version and the original IEEE and
82       The Open Group Standard, the original IEEE and The Open Group  Standard
83       is  the  referee document. The original Standard can be obtained online
84       at http://www.opengroup.org/unix/online.html .
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86       Any typographical or formatting errors that appear  in  this  page  are
87       most likely to have been introduced during the conversion of the source
88       files to man page format. To report such errors,  see  https://www.ker‐
89       nel.org/doc/man-pages/reporting_bugs.html .
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93IEEE/The Open Group                  2017                         SEM_POST(3P)
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